Silver selenide film stoichiometry and morphology control in sputter deposition

ABSTRACT

A method of sputter depositing silver selenide and controlling the stoichiometry and nodular defect formations of a sputter deposited silver-selenide film. The method includes depositing silver-selenide using a sputter deposition process at a pressure of about 0.3 mTorr to about 10 mTorr. In accordance with one aspect of the invention, an RF sputter deposition process may be used preferably at pressures of about 2 mTorr to about 3 mTorr. In accordance with another aspect of the invention, a pulse DC sputter deposition process may be used preferably at pressures of about 4 mTorr to about 5 mTorr.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. patentapplication Ser. No. 10/230,279, filed on Aug. 29, 2002, the disclosureof which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to the field of resistance variable memory devicesformed using a chalcogenide glass and, in particular, to an improvedmethod of depositing a silver-selenide film on a chalcogenide glass.

BACKGROUND OF THE INVENTION

Chalcogenide materials are presently of great interest for use inresistance variable memory devices compared to memory technologiescurrently in use, due to potential advantages in switchingcharacteristics, non-volatility, memory speed, reliability, thermalcharacteristics, and durability. Research in this area is reported inthe articles “High Speed Memory Behavior and Reliability of an AmorphousAs₂S₃ Film doped with Ag” by Hirose et al., Phys. Stat. Sol. (1980),pgs. K187–K190; “Polarity-dependent memory switching and behavior of Agdendrite in Ag-photodoped amorphous As₂S₃ films” by Hirose et al.,Journal of applied Physics, Vol. 47, No. 6 (1976), pgs. 2767–2772; and“Dual Chemical Role of Ag as an Additive in Chalcogenide Glasses” byMitkova et al., Physical Review Letters, Vol. 83, No. 19 (1999), pgs.3848–3851, the disclosures of which are incorporated herein byreference.

In many memory cell designs employing chalcogenide materials, a film ofsilver-selenide (Ag₂Se) is incorporated with a chalcogenide materiallayer. The silver-selenide film is important for electrical performance.Accordingly, silver-selenide deposition is an important aspect offabricating the resistance variable memory device. Most availableresearch in silver-selenide deposition is limited and evaporationdeposition is normally chosen for silver-selenide film formation.

Silver-selenide deposition by evaporation has an attendant problembecause the dissociative properties of silver-selenide make itimpossible to achieve precision stoichiometries of silver-selenide. Itis believed that in evaporation techniques, as the silver starts todiffuse to a lower concentration, it begins to agglomerate. As thesilver is tied up in clusters or agglomerates, selenium is more readilyavailable for evaporation in the beginning of the evaporation process.Thus, during evaporation techniques, selenium is evaporated morequickly, causing the deposition target to become silver-rich. Near theend of the evaporation process little to no selenium is left fordeposition onto the substrate, leaving mostly silver available fordeposition. Thus substantial amounts of selenium are deposited on thesubstrate followed by deposition of primarily silver. Accordingly, theevaporation technique therefore does not uniformly deposit thesilver-selenide and controlling the stoichiometry and surface morphologyof evaporated silver-selenide is difficult.

Furthermore, evaporation deposition is not conducive to industrialapplication. Sputter deposition is more readily available for industrialprocesses and sputter deposition has many advantages compared toevaporation deposition techniques. For example, sputter depositionprovides better film thickness and quality control.

Generally, sputter deposition, or sputtering, is performed by placing asubstrate in a deposition chamber which is evacuated or pressurized to adesired pressure. A particle stream of the film material usuallygenerated from a target is then generated within the chamber and thedeposition occurs by condensation of the particles onto the substrate.In another sputtering technique, often referred to as ion beambombardment sputtering, a high-energy source beam of ions is directedtoward the target. The force of the bombarding ions imparts sufficientenergy to the atoms of the target to cause the energized atoms to leavethe target and form a particle stream. The resulting deposition upon thesubstrate forms a thin film.

Due to the high diffusion property of silver, low melting point ofselenium, and the memory properties of silver-selenide, controlling thestoichiometry and morphology of the silver-selenide film during sputterdeposition is difficult. For instance, silver-selenide bulk material isconductive, but its conductivity (about thousands ohm⁻¹ cm⁻¹) isrelatively lower than that of most metals. Also, silver concentration iscritical for electrical performance of the device, thus it is necessaryto maintain the silver concentration close to about 66.7 atomic weightpercent (herein after represented “%”). With silver concentrationshigher than about 67.5%, many nodular defects are formed in and/or onthe silver-selenide film. The size of these defects can be about a tenthof a micrometer, which could have severe negative impact on sub microndevice fabrication. Although the exact mechanism by which these defectsare formed are unknown, it is believed that these defects are caused byexcess silver, beyond the desired stoichiometric silver concentrationrequirements of the silver-selenide film.

It would be desirable to have an improved method of depositing asilver-selenide film. It would also be desirable to have a method ofcontrolling the stoichiometry and morphology of silver-selenide forsputter deposition.

BRIEF SUMMARY OF THE INVENTION

An exemplary embodiment of the present invention includes a method ofdepositing a silver-selenide film on a substrate. The method includesusing a low pressure sputter deposition process. Preferred sputterdeposition processes include RF sputtering or pulse DC sputtering.Preferably, the sputter deposition will occur in pressures ranging fromabout 0.3 mTorr to about 10 mTorr. The invention is particularly usefulfor depositing a silver-selenide film with better stoichiometricprecision. The invention is also particularly useful for sputterdepositing a silver-selenide film while avoiding nodular defectformation throughout and on the surface of the silver-selenide film.

These and other features and advantages of the invention will be betterunderstood from the following detailed description, which is provided inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is an SEM image of a pulse DC sputter depositedsilver-selenide film deposited using a pressure of 20 mTorr.

FIG. 1( b) is an SEM image of a pulse DC sputter depositedsilver-selenide film deposited using a pressure of 10 mTorr.

FIG. 1( c) is an SEM image of a pulse DC sputter depositedsilver-selenide film deposited using a pressure of 3 mTorr.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to variousspecific structural and process embodiments of the invention. Theseembodiments are described with sufficient detail to enable those skilledin the art to practice the invention. It is to be understood that otherembodiments may be employed, and that various structural, logical andelectrical changes may be made without departing from the spirit orscope of the invention.

The term “silver-selenide” is intended to include various species ofsilver-selenide, including some species which have a slight excess ordeficit of silver, for instance, Ag₂Se, Ag_(2+x)Se, and Ag_(2−x)Se.

The term “chalcogenide glass” is intended to include various compositionstructures based on elements from Group VIA (S, Se, Te, Po, O) alone orin combination with elements from group IV (Si, Ge) and/or group V (P,As, Sb, Bi).

The present invention relates to a process for depositingsilver-selenide. In accordance with the invention, low pressures, of forexample, 0.3 mTorr to about 10 mTorr, are used to sputter depositsilver-selenide. Also in accordance with the invention, silver-selenideis preferably deposited using an RF sputtering process or pulse DCsputtering process.

Silver-selenide itself has electrical memory properties, i.e.conductivity, and sputter deposition processes normally involve strongcurrent, voltage and ion bombardment. Therefore, both electrical andthermal effects from the sputter deposition process can influence thesilver-selenide sputter target and deposited silver-selenide film. Forthe above reason, sputter deposition requires consideration on how toapply electrical power to silver-selenide targets.

Since the conductivity of silver-selenide is relatively lower than thatof most metals, D.C. sputtering has not worked. Regular DC magnetronsputtering attempts have not been effective, primarily because theplasma is not easily ignited.

Depending on the target age, sputter deposition at higher pressures,e.g., about 20 mTorr or greater, result in films with either lower orhigher silver concentrations than the desired stoichiometric silverconcentration of about 66.7%. It has been observed that high pressuredeposition, e.g., about 20 mTorr or greater, of relatively new targetsusing RF or pulse DC magnetron sputter deposition result insilver-selenide films having silver concentrations of only about 60%,which is much lower than the desired stoichiometric silver concentrationof 66.7%. It has also been observed that high pressure deposition, e.g.,about 20 mTorr or greater, of relatively old targets using RF or pulseDC magnetron sputter deposition result in silver-selenide films havingsilver concentrations higher than about 67.5%.

The inventors have discovered that RF or pulse DC magnetron sputterdeposition processes at low pressures ranging from about 0.3 mTorr toabout 10 mTorr may be used to deposit more precise stoichiometricsilver-selenide films while avoiding nodular defects formation in thefilm. It has also been discovered that the silver-selenide targetcomposition changes over the lifetime of the target, and that the use ofa low pressure sputter deposition process allows for precisestoichiometric deposits from both old and new silver-selenide targets.

FIG. 1 shows SEM images of substrates formed of production grade siliconwafers with silicon nitride films having a pulse DC sputter depositedsilver-selenide film of about 500 Angstroms thick. The silver-selenidefilms shown in FIG. 1 were pulse DC sputter deposited using a DentonVacuum Discovery® 24 at 200 kHz with a 1056 ns pulse width, and aconstant power supply of 150 W. A silver-selenide target having astoichiometric silver concentration of about 66.7% was used to depositthe silver-selenide film. Comparing the SEM images of the pulse DCsputter deposited silver-selenide films at various pressures indicatethat low pressure sputter deposition ranging from about 0.3 mTorr toabout 10 mTorr reduces and eliminates nodular defect formations. It wasobserved that a silver-selenide film deposited using high pressure,i.e., about 20 mTorr, has a silver concentration higher than about 67.5%and has nodular defect formations on the surface and through out thefilm as shown in FIG. 1( a); as shown in FIG. 1( b) a depositedsilver-selenide film formed using a low pressure of 10 mTorr hasrelatively few nodular defect formations; and as shown in FIG. 1( c) adeposited film using an even lower pressure of 3 mTorr has a smoothsurface with no nodular defects.

In accordance with a first embodiment of the invention, asilver-selenide target is sputter deposited using an RF sputterdeposition process at a low pressure ranging from about 0.3 mTorr toabout 10 mTorr, and more preferably about 2 mTorr to about 3 mTorr, toprovide a silver-selenide film having little to no nodular defects and asilver concentration of about equivalent to the silver concentration ofa silver-selenide target used to sputter deposit the silver-selenidefilm. For example, where a silver-selenide target having a silverconcentration of about 66.7% is used in the RF sputter depositionprocess, the deposited silver-selenide film will have a silverconcentration of less than about 67.5% and preferably about 67% and morepreferably about 66.7%. A process in accordance with the firstembodiment of the invention may be used for silver-selenide targets ofany age, while still providing a sputter deposited silver-selenide filmhaving a silver concentration about equivalent to that of thesilver-selenide target used to deposit the silver-selenide film.

In a sputtering process in accordance with the first embodiment of theinvention, the sputtering deposition generally takes place in a chamber.An initial base vacuum pressure is established first. The initial basevacuum pressure may be any suitable pressure, including pressures higherthan about 10 mTorr, which may help ignite the plasma. During thesputtering process, process gas should be maintained at a pressureranging from about 0.3 mTorr to about 10 mTorr, and preferably rangingfrom about 2 mTorr to about 3 mTorr. The process gas may be any suitablesputtering process gas, for example, krypton, xenon, helium, neon, argonor combinations thereof. The preferred process gas is argon. Althoughnot wishing to be limited by to any particular amounts of power, powerapplied during the sputtering process preferably may range, for example,between about 100 watts to about 500 watts and is most preferably about150 watts. Power density and power requirements may vary and depend onthe chosen system or size of the target. For example, targets fourinches or larger may require more power. The preferred RF frequency isbetween about 100 kHz and about 20 MHz and is preferably 13.5 MHz. Anexemplary sputter deposition system is the Denton Vacuum Discovery® 24.

In accordance with a second embodiment of the invention, asilver-selenide target is sputter deposited using a pulse DC sputterdeposition process at low pressures ranging from about 0.3 mTorr toabout 10 mTorr to provide a silver-selenide film having a silverconcentration of about equivalent to the silver concentration of asilver-selenide target used to sputter deposit the silver-selenide film.For example, where a silver-selenide target having a silverconcentration of about 66.7% is used in the pulse DC sputter depositionprocess the deposited silver-selenide film will have a silverconcentration of less than about 67.5% and preferably about 67% and morepreferably about 66.7%. A low pressure of from about 4 to about 5 mTorris preferred. There is a difference between RF sputter deposition andpulse DC sputter deposition in that for pulse DC sputtering a depositionpressure of from about 4 to about 5 mTorr, produces a depositedsilver-selenide film having a silver concentration of substantiallyequivalent to the silver concentration of the silver-selenide target,for example 66.7%. However, generally low pressure deposition providessmoother sputter deposited silver-selenide films having a more precisesilver-selenide stoichiometry. The preferred low pressure used may varydepending on the condition of the target, for example, age of thetarget.

Similar to the process described above in accordance, with the firstembodiment of the invention, the sputtering deposition in accordancewith the second embodiment also takes place in a chamber, for example,in a Denton Vacuum Discovery® 24, where a suitable initial base vacuumpressure is established first and a suitable process gas is employed.However, in accordance with the second embodiment, during the sputteringprocess, the process gas should be maintained at a pressure ranging fromabout 0.3 mTorr to about 10 mTorr, and preferably ranging from about 4mTorr to about 5 mTorr. Although not wishing to be limited by to anyparticular amounts of power, the power applied during the sputteringprocess preferably may range, for example, between about 100 watts toabout 500 watts and is most preferably 150 watts and the preferred pulseDC frequency may range, for example, between about 100 kHz and about 250kHz and is preferably about 200 kHz. However, power density and powerrequirements may vary and t will depend on the chosen system and/or sizeof the target. For example, targets four inches or larger may requiremore power. The pulse width should range from about 1000 ns to about1200 ns and is preferably about 1056 ns.

Although the exact mechanism to explain the origin of experimentalobservations is unknown there is a connection between sputter pressure,ion kinetic energy, scattering induced energy reduction, and/or RF andpulse DC plasma electrical properties. For practical application, theinventors propose to use an RF sputter deposition process or pulse DCsputter deposition process at lower pressure to deposit better precisionstoichiometric silver-selenide films and avoid nodular defect formationson the film. Accordingly, pressure may be varied within the low pressurerange of from about 0.3 mTorr to about 10 mTorr to fine tune the silverconcentration of the silver-selenide film. The power sources may bevaried as well. This is of great importance in device fabrication inthat many devices require elemental concentrations slightly deviated(i.e., ±2% at. concentration) from the preferred value of about 66.7%.Accordingly, since low pressure sputter deposition can also be used onrelatively old targets while still providing more precise stoichiometricconcentrations of silver, the invention expands the target lifetime thusreducing process costs.

While exemplary embodiments of the invention have been described andillustrated, various changes and modifications may be made withoutdeparting from the spirit or scope of the invention. Accordingly, theinvention is not limited by the foregoing description, but is onlylimited by the scope of the appended claims.

1. A sputter deposited silver-selenide film comprising silver-selenidehaving the formula of about Ag₂Se and deposited at a pressure of fromabout 0.3 mTorr to about 10 mTorr, said film having substantially nonodular defects.
 2. The film of claim 1 wherein said sputter depositedsilver-selenide film is deposited using a silver-selenide materialhaving a silver concentration of about 66.7 atomic weight percent. 3.The film of claim 2 wherein said silver-selenide film has a silverconcentration of about 67.5 atomic weight percent.
 4. The film of claim1 wherein said silver-selenide film is RF sputter deposited.
 5. The filmof claim 4 wherein said pressure is about 2 mTorr to about 3 mTorr. 6.The film of claim 1 wherein said film is pulse DC sputter deposited. 7.The film of claim 6 wherein said pressure is about 4 mTorr to about 5mTorr.
 8. The film of claim 3 wherein said film has a silverconcentration of about 67 atomic weight percent.
 9. The film of claim 3wherein said film has a silver concentration of about 66.7 atomic weightpercent.
 10. A sputter deposited silver-selenide film having a silverconcentration of between about 67.5 and about 66.7 atomic weight percentand being substantially free of nodular defects.
 11. The film of claim10 wherein said sputter deposited silver-selenide film is depositedusing a silver-selenide target having a silver concentration of about66.7 atomic weight percent.
 12. The film of claim 11 wherein saidsilver-selenide film has a silver concentration of about 67 atomicweight percent.
 13. The film of claim 11 wherein said silver-selenidefilm has a silver concentration of about 66.7 atomic weight percent. 14.The film of claim 10 wherein said sputter deposited silver-selenide isRF sputter deposited.
 15. The film of claim 10 wherein said sputterdeposited silver-selenide is pulse DC sputter deposited.